This invention is directed to light-emitting organic oligomer compositions. This invention is also directed to methods for preparing such oligomer compositions. This invention is further directed to devices comprising films and coatings prepared from such oligomer compositions.
Developments in the field of electroluminescence have resulted in devices having access to the full color spectrum using polymers. These developments have further permitted refinements in device parameters such as brightness, power efficiency, and stability.
In spite of advances in luminescent polymers, these polymers have not been readily adapted for use in liquid crystal displays. It has been suggested that organic light-emitting devices could be employed as backlights in liquid crystal displays, if the devices produced polarized light, because such an application would make the use of polarizers and color filters in current liquid crystal displays redundant. To achieve an organic light-emitting backlight that does not require further processing of light with a polarizer and color filters, a device producing high polarization ratios is needed. One challenge associated with obtaining highly polarized light is obtaining a light-emitting device with highly aligned emitters.
To obtain such a device, organic materials that are highly anisotropic and can be oriented in a specific direction must be employed. A variety of techniques have been employed to obtain light-emitting devices having such orientation, including mechanical alignment, Langmuir-Blodgett deposition, liquid crystalline self-organization and alignment on specific substrates.
Efforts to obtain light-emitting devices having such orientation have involved further manipulation of the organic polymers themselves. Fukuda et al., Japanese Journal of Applied Physics, Vol. 28, pp. L1433–L1435 (1989), discloses polymers and oligomers of fluorenes substituted by alkyl groups at the 9-carbon position. According to Fukuda, fluorene monomers are treated for several days with oxidizing metal salts such as ferric chloride. The resulting structures are poly(fluorene-2,7-diyls). The polymers and oligomers of Fukuda are not satisfactory due to significant crosslinking and mislinking reactions during the polymerization. See Fukuda et al., Journal of Polymer Science, Polymer Chemistry Edition, Vol. 31, pp. 2465–2471 (1993).
Brown et al., Journal of Polymer Science, Polymer Chemistry Edition, Vol. 24, pp. 255–267 (1989), describes chemical defects that are obtained when polyfluorenes are obtained using known techniques. One such problem is that polynuclear structures and substitutions at positions other than the 2,7-positions result. Further, oxidative polymerization can fail to be regiospecific and, thus, fluorenes couple at other positions, such as the 3,5- and 3,6-positions. Also, branching can occur resulting in attachment of more than two fluorene molecules to a given fluorene molecule. Such trifunctional materials can crosslink during preparation of the polymer. In addition, materials produced by known methods can display high polydispersity, tendancy to crystallize, and low glass transition temperatures.
Other fluorenes and preparation techniques have been provided, for example, in U.S. Pat. Nos. 5,708,130, 5,962,631 and 6,169,163 to Woo et al. and U.S. Pat. No. 5,777,070 and PCT Publication No. WO 00/46321 to Inbasekaran et al. However, these approaches do not fully address the difficulties set forth above. The compounds and techniques taught by Woo and Inbasekaran yield polydisperse compositions. Such compositions are limited by the methods by which they can be deposited to create devices. The compositions further suffer from chemical impurities, distributed chain length and distributed chemical composition. Polydisperse oligomer and polymer compositions are not well suited conventional film formation processes. In the instance where vacuum sublimation is employed, because polydisperse compositions comprise molecules having a variety of different molecular weights, an appropriate vapor pressure cannot be selected so that the composition can be evenly applied to a substrate. Polydisperse compositions also do not spin coat well because the wide distribution of high molecular weight constituent molecules results in a non-homogeneous film that is also difficult to align across a large area. Some constituents may crystallize, and aggregates may form. A film formed from polydisperse compositions can include aggregates that are quenching regions, reducing the efficiency of devices formed from such films. Such aggregates can also cause instability in emissive color over time.